PROJECT SUMMARY Neutrophils, eosinophils and ?inflammatory? monocytes collectively account for ~70% of blood leukocytes and are among the shortest-lived cells in the body. Precise regulation of the lifespan of these myeloid cells is critical to maintain effective host responses to pathogens while minimizing the deleterious consequences of prolonged inflammation. However, how the lifespan of these cells is strictly controlled remains largely unknown. This proposal is focused on a novel long non-coding RNA (lncRNA) we recently discovered that acts as a molecular `timer' to control the duration of the lifespan of these inflammatory cells. In my recently established laboratory, we used high-throughput sequencing and a novel bioinformatic workflow to identify Morrbid, a previously uncharacterized lncRNA that is potently and specifically induced by pro-survival cytokines in neutrophils, eosinophils and ?inflammatory? monocytes. Our preliminary data shows that ablation of Morrbid in mice leads to a dramatic reduction in the frequency of these cells at steady state as a result of significant defects in their survival capacity. We found that Morrbid controls cell survival by repressing the transcription of its neighboring pro-apoptotic gene, Bcl2l11 (Bim), in an allelic-specific manner. In addition, we show that Morrbid is conserved in humans, highly expressed in human short-lived myeloid cells and alterations in its levels of expression are associated with hypereosinophilic (HE) pathologies. Thus, defining the molecular mechanisms by which Morrbid controls short-lived myeloid cell lifespan and the role of this novel lncRNA during inflammatory responses will likely reveal new therapeutic targets. As lncRNAs are known to target chromatin modifying complexes to neighboring genes to regulate their expression and Morrbid is potently induced by pro- survival cytokines, our central hypothesis is that Morrbid integrates pro-survival extracellular signals with chromatin modification pathways to control Bcl2l11 expression and thus short-lived myeloid cell survival. Aim 1 of this project will define the molecular mechanism by which Morrbid represses Bcl2l11 expression in response to pro-survival cytokines. In aim 2, we will establish the role of Morrbid in controlling the magnitude of inflammatory responses. For this purpose, we will first use mouse strains with different levels of Morrbid to determine how this lncRNA controls the onset and resolution of eosinophil-mediated lung allergic inflammation. We will then translate these studies to humans as our preliminary data shows that patients with hypereosinophic (HE) pathologies have drastically elevated levels of MORRBID. We will use ex vivo eosinophil culture systems and a humanized mouse model to establish whether MORRBID controls human eosinophil survival. Moreover, using eosinophils from HE patients we will define whether increased MORRBID levels contribute to the development of hypereosinophilia. Collectively, these studies will delineate a novel mechanism by which a lncRNA determines the lifespan of short-lived myeloid cells in response to extracellular cues in mouse and humans. Ultimately, we hope to harness the power of these regulatory mechanisms for therapeutic benefit.